Carbodiimides are a known class of compounds and have found wise use as stabilizers for rubbers, plastics and other materials. Polymeric carbodiimides have found special use as stabilizer against hydrolytic degradation in polyester urethane rubbers. Polymeric carbodiimides are generally formed through the polymerization of a diisocyanate upon treatment with phospholine oxide catalysts. Many of these polycarbodiimides, however, exhibit only limited storage stability through reasons not fully known but believed to be related in some way to the free terminal isocyanate groups remaining on the polycarbodiimide molecule.
The prior art has attempted to improve the storability of polymeric carbodiimides by treating them with a primary or secondary aliphatic alcohol to convert the free isocyanate groups to urethane groups. Specifically, in United States Patent 2,941,983, a urethane-terminated polycarbodiimide is described as being prepared by reacting a diisocyanate or isocyanate terminated linear polymer with a carbodiimide-forming catalyst and controlling the degree of polymerization by monitoring the volume of carbon dioxide given off during the polymerization. Upon reaching a pre-calculated amount of evolved carbon dioxide, polymerization is stopped by introducing a primary or secondary aliphatic alcohol into the reaction mixture to convert the remaining isocyanate groups to urethane groups.
The resulting urethane-terminated polycarbodiimide is limited to a relatively low molecular weight range and requires the utmost care in monitoring the amount of carbon dioxide evolved from the polymerization reaction. Small errors in either the amount of carbon dioxide found to be given off or in the amount of primary or secondary alcohol added to the reaction to terminate the polymerization will lead to either free alcohol groups or free isocyanate groups in the resulting product leading to instability during prolonged storage. In addition, when unsymmetrical arylene diisocyanates such as 2,4-toluene diisocyanate are used in the process, the terminal isocyanate groups are found to be predominantly positioned adjacent the methyl group which sterically hinders the isocyanate group from further reaction.